ABSTRACT
Fibrillarin relies upon a guide RNA and other core proteins in an assembled box C /D RNP to catalyze nucleotide-specific 2'-0-methylation.44,45 Most other methyltranferases utilize accessory domains for substrate specificity. For example, the DNA methylase Hhal recognizes and binds its double-stranded DNA substrate by utilizing a large peripheral domain which binds the DNA and flips the target base out o f the duplex for modification (for details see chapter by Klimasauskas and Liutkeviciute in this book).81 Evolution o f fibrillarin appears to have occurred within the box C /D RNPs as well. Archaeal fibrillarins possess organism-specific NTDs while eukaryotic fibrillarins have related GAR domains.78 Aside from affecting nucleolar localization, the GAR domain serves
Figure 4. Conserved Sequence and Structure of Fibrillarin. A) Sequence alignment of three eukaryotic and three archaeal fibrillarins with the E. coli RrmJ methyltransferase. Degree of conservation is indicated by shades of gray. The highly conserved SAM-binding motif is boxed. B) Crystal structure of M. jannaschii fibrillarin (1FBN). The variable N-terminal domain is light gray, the SAM-binding motif circled and highly conserved catalytic residues designated [black sticks]. C) Spatial superposition of the E. coli RrmJ catalytic residues (black) (1EIZ) with those of M. jannaschii fibrillarin (light gray). The invariant catalytic triad (K-D-K) is labeled and peptide backbones are illustrated with lines.
